The first boss seems to be bugged. When I approach him for the first time, without dying once (on Bad difficulty level), I can put around forty rifle rounds into the SOB and he still wouldn't die. When he inevitably one-shots me (and... yeah, he one-shots me. Is it even possible to survive his attack?) I load the game and he's down in three to four shots. What the hell? is it a glitch? Am I doing something wrong? I swear, it went like this several times.

The game tracks where both your and the enemies' bullets are landing. Headshots are massively more potent than hitting the arms or so. Also you gotta always think about range. I don't remember if you take less damage at a longer range but the chance to be hit will go down dramatically.

BTW if you haven't died at all, you should be playing on the hardest setting. It's very much the intended one for this game.

Try to approach him, crouch in front on the crates next to him and hit him with the knife. As far as I remember, one hit should kill him. Actually even the punch might do.